1. Field of the Invention
A ceramic fiber insulation material is disclosed that is useful in high temperature applications. There is also taught a method for producing the insulation material.
2. Description of the Prior Art
Insulating materials for use in high temperature applications were historically made from asbestos. Besides its excellent thermal insulative properties, asbestos can be manufactured into various geometric shapes for particular applications. For example, annular discs of asbestos can be used to form an outer insulating cover for high temperature roller conveyors. A roller conveyor can be made with an inner metal shaft and an outer insulating cover. The asbestos discs are stacked onto the shaft, and pressed together axially, as discussed, for example, in U.S. Pat. No. 3,802,495. The discs can also be pressed together by hydraulic means as taught in U.S. Pat. No. 3,116,053. Asbestos, in conjunction with a binder, may be formed into numerous shapes besides annular discs, e.g., board product or end caps for furnaces. The health consequences of asbestos exposure has created a need for asbestos-free insulation.
Ceramic fiber has been successfully used as a replacement for asbestos in certain applications. For example, it is taught in U.S. Pat. Nos. 5,205,398 and 5,378,219 to use ceramic fiber as a roll cover. Annular discs of bulk ceramic fiber are stacked onto an inner metal shaft and may be axially compressed at greater than about 50 percent of the fiber's original density. The compressed discs on the metal shaft may be perfused with colloidal silica and dried. The discs are then recompressed and held permanently in compression. Insulating roll covers produced by this process can offer improved insulating properties over asbestos and are substantially free of fugitive binders. The resulting product can have a density above about 40 pounds per cubic foot. A variation of this technology, as described in U.S. Pat. No. 5,355,996, obtains improved wear resistance by periodically sandwiching perforated, rigid, metal discs between the ceramic discs.
Ceramic fibers have also been used as a replacement for asbestos in those areas requiring complex shapes. Fiber is commonly blended with a suitable binder to produce a mixture. A vacuum draws the fiber and colloidal mixture into a mold and substantially removes the binder. A porous fiber product remains. This technology permits ceramic fiber components to be fashioned into various shapes. However, unlike compression techniques, vacuum-forming does not compact the ceramic fibers to a high density and may not imbue the article with desirable strength. The resultant product is of low density, typically no more than 18 pounds per cubic foot. The surface of the product is rigid, but the interior may be weak and friable. Attempts have been made to improve the strength of vacuum-formed articles, e.g., by forming the article around an internal metal mesh.
Despite these improvements in ceramic fiber insulation technology, there is still a need in the industry for an improved, asbestos-free, insulating material, which may be fashioned without compression into a wide variety of shapes yet retain the ruggedness and long service life of asbestos. Current technologies, which compress ceramic fiber, possess excellent properties but cannot easily be fashioned into complex shapes. Vacuum-formed, ceramic fiber articles may be formed into various shapes but lack the strength and durability of articles formed by compression.